Water temperature control for diesel engines

ABSTRACT

Apparatus for controlling the temperature of a coolant in a circulating coolant system for an engine. The temperature of the coolant is sensed at various points in the system by probes which produce an output voltage proportional to temperature. The probe voltages are compared with predetermined &#39;&#39;&#39;&#39;set&#39;&#39;&#39;&#39; voltages so that if a probe&#39;&#39;s voltage exceeds its associated &#39;&#39;&#39;&#39;set&#39;&#39;&#39;&#39; voltage, cooling means, warning means, engine shut down means or other suitable means is actuated.

United States Patent [72] Inventor George Robert Cass Montreal, Quebec,Canada [21 1 Appl. No. 738,699

[22] Filed June 20, 1968 [45] Patented Mar. 9, 1971 [73] AssigneeCanadian National Railway Company Montreal, Quebec, Canada 32 PriorityMay 22, 1968 [33] Canada [54] WATER TEMPERATURE CONTROL FOR DIESELENGINES 24 Claims, 4 Drawing Figs.

52 U.S. Cl 123/41.1s

[5i] lnt.Cl F0lp 11/14 [50] Field ofSearch l23/4l.l5; 236/(TransistorDigest) Primary Examiner-Wendell E. Burns Atlorney-Fetherstonhaugh & Co.

ABSTRACT: Apparatus for controlling the temperature of a coolant in acirculating coolant system for an engine. The temperature of the coolantis sensed at various points in the system by probes which produce anoutput voltage proportional to temperature. The probe voltages arecompared with predetermined set voltages so that if a probes voltageexceeds its associated set voltage, cooling means, warning means, engineshut down means or other suitable means is actuated.

B RELAY 30 l 34 I I l 4 l 5 I AMPLIFIER l l MODULE v J 32 SET POINTMODULE I I \S! PATENTED MR 9 I97! SHEET 1 [If 2 Fig.l.

T lI2K.Q=Rg Rg=lK Q EQUIVALENT GENERATOR q= IMPEDANCE AS RT VARIES DUETo TEMPERATURE THE VOLTAGE Eo IN THE VOLTAGE DIVIDER CIRCUIT VARIESPROPORTION- ATELY TEMPERATURE "F w. m m w w ww 5o S150 Fig.2.

INVENTOR GEORGE R. CASS 'PATENTEDMAR 9m 3568,6 48

SHEET 2 BF 2 ATTORNEYS ZQFUMhOwE CD050 .EOIm mmomm mmh 3 4000 WATERTEMPERATURE CONTROL IFOR DIESEL ENGHNIES This invention relates toapparatus for controlling the temperature of a coolant in a circulatingcoolant system. In particular, it relates to a system for controllingthe temperature of coolant for an internal combustion engine, e.g. in alocomotive.

The purpose of an engine coolant temperature controller on a locomotiveis to control the operation of various cooling functions between thelimits of no cooling and full cooling. If, under full cooling, thecoolant temperature still continues to rise, the controller shouldactivate an alarm to alert the crew, and if a further temperatureincrease occurs, the locomotive should be automatically shut down toprevent engine damage.

The prior art in coolant temperature control consisted of using mercurythermometers modified to act as thermostats. Their contact closure wasthen used to energize or deenergize relays which controlled variouscooling operations within the locomotive. These could be any combinationof louvres (vents) and fans.

The thermometers had probes inserted at various levels so that as themercury column rose it would act as a conductor closing a circuit whichactivated a relay. Problems encountered with this system were mainly dueto the splitting of the mercury column either from vibration encounteredin the locomotive or from voltage transients and current surges producedfrom relays being deenergized and energized.

The present invention utilizes mechanically protected thermistors andelectronic circuitry to activate the various control relays and is morereliable in a locomotive environment. The coolant temperature controlleraccording to the invention controls the operation of various coolingfunctions between the limits of no cooling and full cooling. If,however, under full cooling, the water temperature still continues torise, the controller will activate an alarm to alert the crew. A furthertemperature increase will shut down the locomotive to prevent enginedamage.

According to the invention there is provided apparatus for controllingtemperature of a coolant in a circulating coolant system comprisingprobe means for sensing the temperature of the coolant at a point in thesystem. The probe means produces a voltage substantially proportional tothe coolant temperature. This voltage is applied to one input ofcomparison means having a second input to which is applied a referencevoltage. The comparison means has a first output level when the voltageof the probe is below the reference voltage and a second output levelwhen the voltage of the probe is above the reference voltage. The outputof the comparison means is applied to an input of amplifier meansresponsive to changes in the output level of the comparison means toactuate regulating means for regulating the temperature of the coolant.

The regulating means may be any combination of louvres (vents) and fans.

Preferably a number of comparison means are provided, each comparisonmeans having the probe voltage as one input and a reference voltage asits other input. That is, the reference voltage to each comparator isdifferent and is related to probe temperature so that as the temperatureof the probe rises or falls throughout a certain range, more or fewertemperature regulating means are activated.

Preferably, means are provided for indicating an open circuit of theprobe.

The aforementioned probe is preferably located to sense the temperatureof the coolant just after it leaves the radiator and may be termed acool water probe assuming, for simplicity, that the coolant is simplywater. Naturally it could contain antifreeze, for example. A furtherprobe may be provided to sense the temperature of the water after itleaves the engine and this probe may be termed a hot water probe.

In case the cool water probe should be short circuited, it is preferablethat means be provided so that the hot water probe may take over controland initiate full cooling if it senses a coolant temperatureabove afirst predetermined temperature. If the hot water probe senses atemperature above a second predetermined temperature, it may be arrangedto actuate an alarm to warn the crew that something is amiss. The hotwater probe may also be arranged to shut down the engine or perform someother function if it senses a temperature above a third predeterminedtemperature, i.e. a temperature above which engine damage might occur.

Means are also preferably provided for indicating malfunction of the hotwater probe due to open circuiting of it.

A further probe may be provided to indicate loss of coolant. This probeis provided at a suitable location in the cooling system such that ifthe water goes below this critical level, the engine is in danger ofoverheating. This loss of water probe is identical to the othertwo'probes except that it has a continuously operating heater associatedwith it. All of the probes are preferably of the type employing athermistor bead. Ordinarily, without the heater, if loss of wateroccurred the thermistor bead in the loss of water probe would onlyprovide an output proportional to the temperature of the surrounding airin the pipe. With the heater, and water at normal levels the heater hasvery little effect on the thermistor bead temperature because it isimmersed in continuously circulating water. When loss of water occurs,the heater is able to heat the thermistor bead to a level sufficient toactivate a comparator module denoting loss of water in the coolingsystem and the engine is shut down.

The invention will now be further described in conjunction with theaccompanying drawings, wherein;

FIG. 1 shows a probe which may be used in the coolant flow controlsystem of the present invention;

FIG. 2 is a graph showing output voltage versus temperature for athermistor which may be used in the probe of FIG. 1;

FIG. 3 is a schematic diagram of a comparator module, set point moduleand amplifier module used in the system according to the invention; and

FIG. 4 is a detailed, partly block, partly schematic, diagram of asystem according to the invention for controlling the temperature ofcoolant in a circulating coolant system.

The construction of the probes is shown in FIG. 1. The probe comprises ahollow metal tube 10 having a closed end 12 and an open end 13. Athermistor bead 14 is contained in the hollow tube 10 adjacent theclosed end 12 and has a pair of leads 15 extending from the open end 13of the tube. The tube 10 is supported within a body member 20 having anexternally threaded end 21 adapted to cooperate with an internallythreaded opening in the coolant system (not shown) whereby the bodymember 20 may be mounted with the closed end 12 of the tube 10 extendinginto the coolant. Seals are provided as shown to prevent leakage ofcoolant along the tube 10. The heater is provided only on the loss ofwater probe for a purpose which will be made clear subsequently herein.

It will be appreciated that the thermistor is mechanically protectedfrom the cooling water (which may contain corrosive agents) by the tube10 so that water may not contact the thermistor. The tube 10 ispreferably of metal, e.g. brass and may be packed with silicone greaseso that variations in the temperature of the coolant are rapidlytransmitted to the thermistor bead 14.

FIG. 2 shows the variation of the thermistors characteristics withtemperature. The upper right hand portion of FIG. 2 shows the thermistorconnected in series with a resistor, e.g. 2K% to form a voltage dividercircuit. An output voltage E may be taken across the 2K% resistor.Variations in resistance of the thermistor R result in variations of theoutput voltage E across the 2K% resistor.

FIG. 3 shows three basic modules suitable for use in the presentinvention, a comparator module 30, a set point" module 31 and anamplifier module 32.

Resistors R R and R in the set point module 31 are used to set areference voltage proportional to temperature (see FIG. 2) for thedifference amplifier comprising transistors Q and Q, in the comparatormodule 30. if the voltage level from the probe to the base of transistor0, is lower than the reference voltage applied to the base of transistorQ by the set point module 31 (over line 33), then 0, will be conductingand transistor Q will be cut off. The collector of transistor Q. isconnected to the base transistor so that when transistor Q. isconducting, it will cause transistor O to conduct. Conduction oftransistor Q will allow current to flow into the base of transistor Q,in the amplifier module 32 thus holding it in a conducting state andthus holding the relay 34 continuously energized unless the probevoltage applied to the base of Q rises, due to an increase in coolanttemperature, to a voltage equal to or slightly greater than that appliedto the base of Q by the set point module 31.

First and second control resistors R and R,, are connected between thecollectors of transistors Q and Q respectively, and supply terminal 1 asshown.

The relay 34 has contacts (not shown) which are open when the relay coilis energized and closed when it is deenergized. This provides forfail-safe operation in that the contacts close regardless of the reason(for example battery failure) for deenergization of the relay coil.

The output of transistor 0 also provides a hysteresis feature when itconducts into resistor R,, Due to the current flowing through R,,, thepotential at point 1 of the set point module 31 is higher than itnormally would be if transistor 0,, were not conducting.

As the temperature of the probe rises to a point where Q begins toconduct, transistor Q, also begins conducting while transistors Q, and Qcease conduction. With transistor Q off, the potential at point 5 of thecomparator module 30 becomes open circuit thus lowering the potential atpoint 1 of the set point module 31. This lowering of potential at point1 of the setpoint module 31 occurs because current due to the potentialat point 2 of the set point module 31 ceases flowing through resistorR,,, and the lower portion of the potentiometer, i.e. below theadjustable tap. This lowering of potential at point 1 of the set pointmodule 31 further ensures that transistor Q stays conducting and Q,stays cutoff. The lowering of potential at point 1 of the set pointmodule 31 can be set at any value depending on resistor choice,'normallyset at a voltage equivalent to about 3 F. Thus the probe voltage E,,,,as a result of cooling, must drop a certain amount below the value whichwas initially required to turn on transistor 0;. This hysteresis featureprevents abnormally fast cycling of a specific cooling operation.

When transistor 0 ceases conduction, the base drive of the normallycontinuously conducting transistor O is removed to that transistor Qstops conducting thus deenergizing relay 34 which brings on a specificcooling operation. That is, the relay 34 controls equipment (not shown)for influencing the rate of cooling of the coolant.

The resistor R,,, in the set point module 31 allows the potential atpoint 1 in the set point module 31 to be raised or lowered by apredetermined amount. Thus several degrees of cooling must occur beforethe cooling operation is stopped and conversely several degrees ofheating must occur before the cooling operation again begins. The diodesD, and D in the set point module 31 are used to prevent the potential atpoints 3 and 1 of the set point module from affecting the operation ofthe amplifier module 32. These potentials would otherwise be presenteddirectly into the base of transistor 0 which would then conductcontinuously regardless of the output from the comparator module 30.

The network preceding transistor 0, in the amplifier module 32 servesthree purposes:

a. Resistor R provides a slight off bias for the base of Q ensuring thatwhen transistor 0, is required to be nonconducting it will stay thatway;

b. resistor R is a current limit for the base drive of transistor 0,;and

c. resistor R and capacitor C, provide a high frequency noise filter.

Diode D in parallel with the relay 34 prevents any large voltagetransients induced during deenergization of the relay from exceeding thesustaining voltage of transistor 0, and thus destroying it.

The various terminals of the modules have been numbered in FIG. 3. InFIG. 4, the modules are shown in block form only but a number of theterminals have been provided with numbers which correlate with theterminal numbers given in FIG. 3. For production efficiency, each of thecomparator modules are preferably made identical, the same being truefor the set point and amplifier modules. The number of modules used area function of the degree of temperature control required. FIG. 4 is adiagram of a three-probe, fail-safe water temperature controller butthis may easily be simplified to a one-probe three-relay operation.

Terminal 4 is only used in amplifier modules A,,, A,, and A,, andterminal 5 is only used in amplifier module A Terminal 4 of thesemodules provides a near ground potential through saturated transistor 0,for other amplifier modules. Amplifier module A,, provides a ground forA,, amplifier A,, provides a ground for A,,, and amplifier A,, providesa ground for amplifiers A,, A A and A,. If Q, and O in FIG. 4 groundterminal 5 of amplifier A,, the function of comparator C is nullifiedbecause the base of Q, in amplifier module A,, is then grounded.

The arrangement shown in FIG. 4 comprises a cool water probe 41 forsensing the temperature of coolant at a point in a cooling system, eg acooling system of a locomotive. The probe means 41 produces a voltagesubstantially proportional to the coolant temperature and this voltageis applied to one input of comparison means having a second input towhich is applied a reference voltage. In this case, the voltage of probe41 is applied to one input of each of comparator means C,, C C C and C,,Comparators C, to C each have a reference voltage applied as a secondinput (on their number 3 terminal) by set point circuits SP, to SP.,,respectively. Each comparator C, to C has a first output level (high)when the voltage of the probe 41 is below the reference voltage and asecond output level (low) when the voltage of the probe is above saidreference voltage. Each comparator means C, to C, has its outputconnected to an associated amplifier A, to A Normally these amplifiersare conducting but when the probe voltage exceeds the set point" voltageof a comparator, it cuts off the associated amplifier which releases theassociated relay R,, R R or R The reference voltages applied to thecomparators C, to C are set by the set point circuits SP, to SP atdifferent levels corresponding to different coolant temperature. Thus,relay R,, may be activated at a coolant temperature of 148 F., relay R,,at a temperature of 152 F., relay R, at a temperature of 155 F. andrelay R, at a temperature of F. These relays R, to R,, may actuate anycombination of louvres (vents) and fans.

The comparator C,, has an input from the cool water probe 41 and, as areference input, a voltage from the circuit generally indicated as 42which applies a low voltage of, for example, 0.20 volts. Throughout thenormal range of operation of the probe, the voltage of the probe 41always exceeds the reference voltage applied to comparator C,, to thatcomparator C,, has a low level output insufficient to drive amplifierA,,. However, should the probe become open circuited, its voltage woulddrop to zero so that comparator C,, would change its output from a lowoutput level to a high output level thus turning on amplifier A,, andhence turning on the probe loss indicator light No. 1.

The reference voltage for each comparison means C, to C, is manuallyadjustable by adjusting the tap on the potentiometer R,, (FIG. 3).

The cool water probe 41 is responsive to the temperature of coolantafter passage through a radiator (not shown) forming part of a coolantsystem for an internal combustion engine, e.g. in a locomotive.

A further probe 43 is provided which is responsive to the temperature ofthe coolant after heating by the engine and hence may be termed a hotwater probe. The output of this probe is connected to comparator means CC C and C If the hot water probe detects a temperature in excess of afirst predetermined temperature, e.g. 175 F., the output of comparatormodule C drops so that amplifier A cuts off. This raises the potentialon terminal 4 of amplifier A and this is ap plied to terminal 3 of eachof amplifiers A to A, so that relays R, through R, are deactivated andhence cause maximum cooling to occur.

it the temperature detected by hot water probe 43 is still higher, e.g.180 F., the output of comparator C drops and relay R is released toactivate an alarm to warn the locomotive crew that something is wrong.

Should the temperature detected by the hot water probe 43 exceed a thirdpredetermined temperature, e.g. 185 F., the output of comparator C dropsand relay R is released. This can be arranged to cause engine shutdownor perhaps some other action not quite as drastic as engine shutdown bymeans not shown but well known to those skilled in the art so thatdetailed explanation is believed unnecessary. For example relay R couldsimply actuate means to stop the supply of fuel to the engine.

The reference voltage for comparator C is provided by the same circuitryindicated generally at 42 that provides a reference voltage forcomparator C so that if the hot water probe 43 becomes open circuited,the output of comparator C rises so that an output appears on amplifierA and turns on probe loss indicator light No. 2.

The set point circuit SP applies a reference voltage to a comparator C,as well as to the comparator C The other input of comparator C as wellas to the comparator C The other input of comparator C is from the coolwater probe 41. If probe 41 short circuits it causes a sharp increase inthe input voltage to comparator C, so that its output drops to a lowlevel. This cuts off amplifier A which applies an increases voltage fromits terminal 4 to the terminal 3 of amplifier A This cuts off amplifierA which in turn releases relay R to cause engine shutdown, for example.

When hot water probe 43 is functioning properly, current flows fromterminal 6 of comparator module C to the negative voltage source V, viaresistor R This causes a positive bias to be applied to the base of NPNtransistor Q so that it is conducting. The collector of transistor O isconnected to the base of NPN transistor which has its emitter groundedand its collector connected to terminal of amplifier module A Whentransistor Q is conducting, Q is cut off. However, if probe 43 shouldbecome open circuited, an output would appear on terminal 5 of thecomparator module C whereas the output on terminal 6 would disappearhence removing the bias from the base of the transistor Q, Which wouldthen cut off. The resulting increase in the potential at the collectorof Q would be applied to the base of transistor Q making it conductingand hence effectively grounding terminal 5 of amplifier A This groundingof terminal 5 of amplifier A prevents it from operating, i.e. cuts itsoff thus releasing relay R to shut down the engine.

A further probe 44, termed a loss-of-water probe, may be provided at asuitable location in the cooling system, such that if the water goesbelow this critical level, the engine is in danger of over heating. Thisprobe is provided with a heater 45 (see also FIG. 1). This probe isidentical to the other two probes except for the heater which iscontinuously operating and provides low heating power, e.g. 8 watts.Ordinarily, without this heater, if loss of water occurred, thethermistor head would only provide an output proportional to thetemperature of the surrounding air in the pipe in which it is mounted.With the heater on, and water at normal level, the heater has verylittle effect on the bead temperature since it is immersed incontinuously circulating water. When loss of water occurs the heater isable to heat the thermistor head to a level sufficient to activate acomparator module C denoting loss of water in the cooling system.Activation of comparator module C releases a relay R, which causesengine shutdown or some other function in a similar manner as relay REngine shutdown, or perhaps some other less drastic action, also occursupon short circuiting of the loss of water probe 44 because this alsoresults in an increased input on terminal 2 of comparator module C Theoutput of the probe 44 is also applied to terminal 2 of a comparator Chaving as a reference voltage the voltage from the circuitry 42. Thevoltage from circuitry 42 is less than the voltage normally provided byprobe 44 so that comparator module C produces an output on its terminal6 which is applied to terminal 1 of amplifier A which is thereforeconductive. Terminal 4 of amplifier A is connected to terminal 3 ofamplifier A, to provide a ground for amplifier A Should the loss ofwater probe 44 become open circuited, the voltage on terminal 2 ofcomparator C would disappear and the output on terminal 6 of comparatorC would disappear so that amplifier A would cut off eliminating theground connection for amplifier A, which then would cut off releasingrelay R to shut down the engine and activate probe loss indicator lightNo. 3. The various voltages V,,, V;,, V 0 (ground) and V are provided bya power supply 50 connected through a network 51 to a battery 52, e.g..a72 volt battery on a locomotive. The details of the power supply 50 arenot pertinent to the present invention and hence are not shown. Ifdesired, separate supplies could be provided for the various voltagesused in the circuitry of the invention. The network connected betweenthe battery and power supply is used for voltage dropping, e.g. fromplus 72v. to plus 1 2v., and also for transient voltage filtering.

The probes 41, 43 and 44 may be provided with networks 60, 61 and 62,respectively, for smoothing purposes to nullify any voltage transientswhich might be induced in the circuitry during normal locomotiveoperation. Electrolitic capacitors may-be used in the networks 60, 61and 62.

, Iclaim:

1. Apparatus for controlling temperature of a coolant in a circulatingcoolant system comprising probe means for sensing the temperature ofsaid coolant at a point in said system, said probe means producing avoltage substantially proportional to said temperature, said voltagebeing applied to one input of comparison means having a second input towhich is applied a reference voltage, said comparison means having afirst output level when the voltage of said probe is below saidreference voltage and a second output level when the voltage of saidprobe is above said reference voltage, the output of said comparisonmeans being applied to an input of amplifier means responsive to changesin the output level of said comparison means to actuate regulating meansfor regulating the temperature of said coolant.

2. Apparatus as claimed in claim 1 wherein the voltage of said probe isalso applied as an input to a plurality of additional comparison meanseach having a second input to which is applied a different referencevoltage, each comparison means having a first output level when thevoltage of said probe is below the reference voltage applied to itssecond input and a second output level when the voltage of said probe isabove the reference voltage applied to its second input, the output ofeach comparison means being applied to an input of a different amplifiermeans, there being a different amplifier associated with each comparisonmeans, each amplifier means being responsive to changes in the outputlevel of its associated comparison means to actuate regulating meansassociated therewith for regulating the temperature of said coolant.

3. Apparatus as claimed in claim 2 wherein the reference voltage foreach comparison means is manually adjustable.

4. Apparatus as claimed in claim 2 wherein each different referencevoltage is related to a different temperature of said coolant.

5. Apparatus as claimed in claim 2 wherein a further comparison means isprovided having the voltage of said probe applied to one input and areference voltage applied to a second input, the reference voltageapplied to the second input of said further comparison means being lowerthan any range of voltages adapted to be produced by said probe whensensing the temperature of said coolant, said further comparison meansproducing an output unless the output from said probe is interrupted inwhich case its output ceases and causes actuation of a warningmechanism.

6. Apparatus as claimed in claim 2 wherein the reference voltage foreach comparison means is provided by a set point circuit, there being adifferent set point circuit associated with each comparison means, eachset point circuit having means for introducing a hyteresis function inthe voltage applied to the second input of its associated comparisonmeans whereby the output of its associated comparison means changes fromsaid first level to said second level at a value of probe voltageslightly higher than a value of probe voltage which will cause theoutput of the associated comparison means to change from said secondlevel to said first level.

7. Apparatus as claimed in claim 6 wherein each set point circuitcomprises a potentiometer having a substantially constant voltageapplied across it, said potentiometer having an adjustable tap connectedto the second input of its associated comparison means.

8. Apparatus as claimed in claim 7 wherein each said set point circuitincludes means for increasing the voltage at said adjustable tap whenthe output of its associated comparison means isat said second level.

9. Apparatus as claimed in claim 8 wherein said potentiometer has afirst end connected to a positive DC voltage source and a second endconnected to ground and the means for increasing the voltage at saidadjustable tap when the output of its associated comparison means is atsaid second level comprises a first diode and a second diode, eachhaving an anode and a cathode, said first diode having its anodeconnected to the anode of said second diode and to the output of theassociated comparison means, the cathode of said first diode beingconnected to the first end of said potentiometer and the cathode of saidsecond terminal connected to said adjustable tap.

10. Apparatus as claimed in claim 1, wherein said probe comprises ahollow elongated metal tube having a closed end and an open end, saidclosed end being adapted to be inserted in said coolant, a thermistorbeing contained in said tube adjacent said closed end and havingelectrical leads extending from said open end.

11. Apparatus as claimed in claim 4, wherein said probe comprises ahollow elongated metal tube having a closed end and an open end, saidclosed end being adapted to be inserted in said coolant, a thermistorbeing contained in said tube adjacent said closed end and havingelectrical leads extending from said open end.

12. Apparatus as claimed in claim 7, wherein said probe comprises ahollow elongated metal tube having a closed end and an open'end, saidclosed end being adapted to be inserted in said coolant, a thermistorbeing contained in said tube adjacent said closed end and havingelectrical leads-extending from said open end.

13. Apparatus as claimed in claim 2 wherein the coolant system isadapted to cool an internal combustion engine and said coolant is heatedby said engine and cooled by a radiator and said probe is responsive tothe temperature of the coolant after passage through said radiator.

14. Apparatus as claimed in claim 13 comprising a further proberesponsive to the temperature of said coolant after heating by saidengine, said further probe being connected to means adapted to causefull cooling of said engine upon detection of a temperature above afirst predetermined temperature and to means adapted to actuate or alarmupon detection of a temperature above a second predetermined temperatureand to means adapted to shut down the engine upon detection of atemperature above a third predetermined temperature.

15. Apparatus as claimed in claim 14 comprising a third probe adapted todetect loss of coolant from said coolant system below a predeterminedminimum, said third probe being connected to means for shutting downsaid engine upon detection of loss of coolant below said predeterminedminimum or upon short circuiting or open circuiting of said third probe.

16. Apparatus as claimed in claim 15 including means for indicating opencircuiting of said further probe.

17. Apparatus as claimed in claim 15 wherein said third probe isprovided with an electrical heater, said heater producing insufficientheat to influence said third probe when it is immersed in coolant butcausing said third probe to produce an output when said third probe isnot immersed in coolant.

18. Apparatus as claimed in claim 2, wherein said probe comprises ahollow elongated metal tube having a closed end and an open end, saidclosed end being adapted to be inserted in said coolant, a thermistorbeing contained in said tube adjacent said closed end and havingelectrical leads extending from said open end.

19. Apparatus as claimed in claim 3, wherein said probe comprises ahollow elongated metal tube having a closed end and an open end, saidclosed end being adapted to be inserted in said coolant, a thermistorbeing contained in said tube adjacent said closed end and havingelectrical leads extending from said open end.

20. Apparatus as claimed in claim 5, wherein said probe comprises ahollow elongated metal tube having a closed end and an open end, saidclosed end being adapted to be inserted in said coolant, a thermistorbeing contained in said tube adjacent said closed end and havingelectrical leads extending from said open end.

21. Apparatus as claimed in claim 6, wherein said probe comprises ahollow elongated metal tube having a closed end and an open end, saidclosed end being adapted to be inserted in said coolant, a thermistorbeing contained in said tube adjacent said closed end and havingelectrical leads extending from said open end.

22. Apparatus as claimed in claim 8, wherein said probe comprises ahollow elongated metal tube having a closed end and an open end, saidclosed end being adapted to be inserted in said coolant, a thermistorbeing contained in said tube adjacent said closed end and havingelectrical leads extending from said open end.

23. Apparatus as claimed in claim 9, wherein said probe comprises ahollow elongated metal tube having a closed end and an open end, saidclosed end being adapted to be inserted in said coolant, a thermistorbeing contained in said tube adjacent said closed 'end and havingelectrical leads extending from said open end.

24. Apparatus as claimed in claim 16 wherein said third probe isprovided with an electrical heater, said heater producing insufficientheat to influence said third probe when it is immersed in coolant butcausing said third probe to produce an output when said third probe isnot immersed in coolant.

1. Apparatus for controlling temperature of a coolant in a circulating coolant system comprising probe means for sensing the temperature of said coolant at a point in said system, said probe means producing a voltage substantially proportional to said temperature, said voltage being applied to one input of comparison means having a second input to which is applied a reference voltage, said comparison means having a first output level when the voltage of said probe is below said reference voltage and a second output level when the voltage of said probe is above said reference voltage, the output of said comparison means being applied to an input of amplifier means responsive to changes in the output level of said comparison means to actuate regulating means for regulating the temperature of said coolant.
 2. Apparatus as claimed in claim 1 wherein the voltage of said probe is also applied as an input to a plurality of additional comparison means each having a second input to which is applied a different reference voltage, each comparison means having a first output level when the voltage of said probe is below the reference voltage applied to its second input and a second output level when the voltage of said probe is above the reference voltage applied to its second input, the output of each comparison means being applied to an input of a different amplifier means, there being a different amplifier associated with each comparison means, each amplifier means being responsive to changes in the output level of its associated comparison means to actuate regulating means associated therewith for regulating the temperature of said coolant.
 3. Apparatus as claimed in claim 2 wherein the reference voltage for each comparison means is manually adjustable.
 4. Apparatus as claimed in claim 2 wherein each different reference voltage is related to a different temperature of said coolant.
 5. Apparatus as claimed in claim 2 wherein a further comparison means is provided having the voltage of said probe applied to one input and a reference voltage applied to a second input, the reference voltage applied to the second input of said further comparison means being lower than any range of voltages adapted to be produced by said probe when sensing the temperature of said coolant, said further comparison means producing an output unless the output from said probe is interrupted in which case its output ceases and causes actuation of a warning mechanism.
 6. Apparatus as claimed in claim 2 wherein the reference voltage for each comparison means is provided by a set point circuit, there being a different set point circuit associated with each comparison means, each set point circuit having means for introducing a hyteresis function in the voltage applied to the second input of its associated comparison means whereby the output of its associated comparison means changes from said first level to said second level at a value of probe voltage slightly higher than a value of probe voltage which will cause the output of the assOciated comparison means to change from said second level to said first level.
 7. Apparatus as claimed in claim 6 wherein each set point circuit comprises a potentiometer having a substantially constant voltage applied across it, said potentiometer having an adjustable tap connected to the second input of its associated comparison means.
 8. Apparatus as claimed in claim 7 wherein each said set point circuit includes means for increasing the voltage at said adjustable tap when the output of its associated comparison means is at said second level.
 9. Apparatus as claimed in claim 8 wherein said potentiometer has a first end connected to a positive DC voltage source and a second end connected to ground and the means for increasing the voltage at said adjustable tap when the output of its associated comparison means is at said second level comprises a first diode and a second diode, each having an anode and a cathode, said first diode having its anode connected to the anode of said second diode and to the output of the associated comparison means, the cathode of said first diode being connected to the first end of said potentiometer and the cathode of said second terminal connected to said adjustable tap.
 10. Apparatus as claimed in claim 1, wherein said probe comprises a hollow elongated metal tube having a closed end and an open end, said closed end being adapted to be inserted in said coolant, a thermistor being contained in said tube adjacent said closed end and having electrical leads extending from said open end.
 11. Apparatus as claimed in claim 4, wherein said probe comprises a hollow elongated metal tube having a closed end and an open end, said closed end being adapted to be inserted in said coolant, a thermistor being contained in said tube adjacent said closed end and having electrical leads extending from said open end.
 12. Apparatus as claimed in claim 7, wherein said probe comprises a hollow elongated metal tube having a closed end and an open end, said closed end being adapted to be inserted in said coolant, a thermistor being contained in said tube adjacent said closed end and having electrical leads extending from said open end.
 13. Apparatus as claimed in claim 2 wherein the coolant system is adapted to cool an internal combustion engine and said coolant is heated by said engine and cooled by a radiator and said probe is responsive to the temperature of the coolant after passage through said radiator.
 14. Apparatus as claimed in claim 13 comprising a further probe responsive to the temperature of said coolant after heating by said engine, said further probe being connected to means adapted to cause full cooling of said engine upon detection of a temperature above a first predetermined temperature and to means adapted to actuate or alarm upon detection of a temperature above a second predetermined temperature and to means adapted to shut down the engine upon detection of a temperature above a third predetermined temperature.
 15. Apparatus as claimed in claim 14 comprising a third probe adapted to detect loss of coolant from said coolant system below a predetermined minimum, said third probe being connected to means for shutting down said engine upon detection of loss of coolant below said predetermined minimum or upon short circuiting or open circuiting of said third probe.
 16. Apparatus as claimed in claim 15 including means for indicating open circuiting of said further probe.
 17. Apparatus as claimed in claim 15 wherein said third probe is provided with an electrical heater, said heater producing insufficient heat to influence said third probe when it is immersed in coolant but causing said third probe to produce an output when said third probe is not immersed in coolant.
 18. Apparatus as claimed in claim 2, wherein said probe comprises a hollow elongated metal tube having a closed end and an open end, said closed end being adapted to be inserted in said coolant, a thermistor being contained in said tube adjacent said closed enD and having electrical leads extending from said open end.
 19. Apparatus as claimed in claim 3, wherein said probe comprises a hollow elongated metal tube having a closed end and an open end, said closed end being adapted to be inserted in said coolant, a thermistor being contained in said tube adjacent said closed end and having electrical leads extending from said open end.
 20. Apparatus as claimed in claim 5, wherein said probe comprises a hollow elongated metal tube having a closed end and an open end, said closed end being adapted to be inserted in said coolant, a thermistor being contained in said tube adjacent said closed end and having electrical leads extending from said open end.
 21. Apparatus as claimed in claim 6, wherein said probe comprises a hollow elongated metal tube having a closed end and an open end, said closed end being adapted to be inserted in said coolant, a thermistor being contained in said tube adjacent said closed end and having electrical leads extending from said open end.
 22. Apparatus as claimed in claim 8, wherein said probe comprises a hollow elongated metal tube having a closed end and an open end, said closed end being adapted to be inserted in said coolant, a thermistor being contained in said tube adjacent said closed end and having electrical leads extending from said open end.
 23. Apparatus as claimed in claim 9, wherein said probe comprises a hollow elongated metal tube having a closed end and an open end, said closed end being adapted to be inserted in said coolant, a thermistor being contained in said tube adjacent said closed end and having electrical leads extending from said open end.
 24. Apparatus as claimed in claim 16 wherein said third probe is provided with an electrical heater, said heater producing insufficient heat to influence said third probe when it is immersed in coolant but causing said third probe to produce an output when said third probe is not immersed in coolant. 